My understanding is that the NowTV Broadband & Telephone service and TV passes would be separate and you won't be able to combine them together (not sure how the old legacy Combo service worked).
If you haven't got an active month sports pass then you could wait until the same date has your Broadband & Telephone bill to buy the month sports pass which should then be paid on the same date has your Broadband bill but it will be a separate charge of £33.99 per month (unless NowTV offer extra discount which i am not aware of).
Where the sports monthly pass will keep auto renewing each month until you cancel.
If i was going down the monthly sports pass route and you can afford the £199 up front payment then maybe consider the 10 month £199 offer which ends on the 26th of this month.
Just a word of caution a lot of people seem to be having trouble buying or activating the 10 month pass on their account for some unknown reason.
Or maybe consider £25 one month sports pass Gift Cards from Argos, Currys or NowTV website linked below which you can apply to your account.